A variety of fixation devices for the reduction of bone or bone fragments are well known. For instance, external bone fixation devices, or external fixators, are used to reduce fractures of the long bones in the human body. Internal bone fixation devices, such as bone plates, are also commonly used to reduce bone fractures. Spinal fixation devices including intervertebral implants, spinal rods, and the like, are used to replace intervertebral discs, fuse or align adjacent vertebrae, and address other spinal issues.
A large number of fixation devices are attached to underlying bone using bone anchors, which can include screws, pins, nails, and the like. For instance, a typical bone plate includes screw holes that accommodate bone screws which are drilled into underlying bone on opposing sides of a fracture to join bone segments together. A typical cervical spine implant can likewise include screw holes that accommodate screws which are drilled into adjacent vertebral bodies in order to fix the position of the implant. Unfortunately, the attachment of fixation devices to the underlying bone can become compromised if, for instance, the screw becomes dislodged from the bone during normal anatomical function.
Referring to FIGS. 1A-B, a conventional anchor-in-anchor fixation system 20 includes a first bone anchor 22 that includes a first head 28 and a first shaft 26 that extends from the first head 28 and is integral and monolithic with the first head 28, and a second bone anchor 24 that includes a second head 44 and a second shaft 42 that extends from the second head 44 and is integral and monolithic with the second head 44. The first bone anchor 22 defines a bore 40 that extends through the first head 28 along a direction oblique to the first shaft 26. The bore 40 is sized greater than the second shaft 42, such that the second shaft 42 passes through the bore 40. The bore 40 can be threaded, and sized substantially equal to the second head 44, which can also be threaded, such that the second head 44 threadedly mates with the first head 28 in the bore 40. Thus, the second shaft 42 extends along a direction oblique with respect to the first shaft 26, which creates a stable triangular load bearing plane that allows the anchor-in-anchor fixation system 20 to withstand higher forces and prevent subsidence or migration with respect to single anchors.
As illustrated in FIG. 1D, the anchor-in-anchor fixation system 20 can join a pair of bone fragments 45a and 45b of a fractured bone 45 together, for instance when repairing the fractured bone 45, and can further fix an implant to the bone 45. The bone 45 can be a long bone, the first bone fragment 45a can be a diaphysis, or shaft, of the long bone, and the second bone fragment 45b can be a metaphysis of the long bone, though it is appreciated that the bone 45 can be any suitable bone as desired. As illustrated in FIG. 1C, the anchor-in-anchor fixation system 20 can be used to attach an implant to the bone 45. In particular, one of the bone anchors, such as the first bone anchor 22, can be driven through a bone plate 47 and into the bone 45, and the second bone anchor 24 can be driven through the bore 40 of the first head 28 and into the bone 45. As illustrated in FIG. 1D, the anchor-in-anchor fixation system 20 can be used to attach an implant in the medullary canal of the bone 45. In particular, one of the bone anchors, such as the first bone anchor 22, can be driven into the bone 45 and through an aperture of an intramedullary nail 49, and the second bone anchor 24 can be driven through the bore 40 of the first head 28 and into the bone 45.